This post is about temporary repairing the cylinder liner in case of scoring which if is detected at an early stage and if appropriate countermeasures are taken, sudden severe wear (scuffing) , losing liner and piston rings, can be avoided and the reliability improved. Visual piston underside inspection is recommended to be carried out at every four to eight weeks.
Even though the surface appears to be in good condition at first glance, the rough surface and sharp vertical scars generated by foreign particles and minor abrasion between liner and piston rings may emerge partially in the cylinder liner. In the worst-case scenario, this will result in excessive wear on the cylinder liner and piston rings (scuffing). It is therefore recommended to inspect the liner surface carefully whenever opening the cylinder cover. If the liner surface is rough and has sharp vertical scars, this must be dressed up by using hand-honing and it is especially recommended when burnt markings and/or deep vertical scars on the surface of piston rings are discovered, since this indicates the similar condition on the cylinder liner surface.
For such kind of scoring marks, the intention is to break the surface of such scored liner structure and not to remove them completely as this might result in a liner shape that is too uneven and causes blow-by.
The tool necessary for hand-honing comprises of:
- angle grinder – 100 mm or 180 mm diameter
- emery cloth disc – grit size #80 and #120
In order to perform a correct hand-honing you must take into the consideration the following procedure:
- Hand-honing must be applied to the liner surface in the whole circumference and full length of piston stroke as much as possible.
- If the piston is installed, it needs to be cover up to avoid dirt ingress between piston, piston rings and liner.
- The cylinder liner surface should be obliquely honed (WW shape)
- The sharp ridges should be honed half of their height. The honing should be finished when the liner surface become smooth (e.g. no cotton fiber stuck on surface).
- In case of extremely hard, blue colour patches all protruding lines have to be grinded. The dressing up should be done by disk cutter, followed by polishing with emery cloth disc. The disc cutter, of 100 – 250 mm diameter and maximum thickness of 2 mm, should be used in the flattest possible angle to obtain a smooth surface.
- Clean all around the liner after honing by wiping the cylinder liner surface, cylinder oil groove and scavenging ports.
In conclusion, cylinder liner dress up is a temporary and an emergency countermeasure used in order to save the liner. Normally in these cases, cylinder liner need to be removed and re-honed in a specialized workshop or in-situ onboard by utilizing a special honing tool which is not easily available for large bore engines. The job is time consuming and requires adequate skills.
However, before honing the cylinder liner must fulfil few conditions:
- No cracks are present
- If the liner wear is eccentric and more than 1 mm out of centre, then honing is not recommended. This is difficult to measure, but one method is to compare the thickness of the wear edges around the liner.
- From TDC of the top piston ring wear edge to about mid-stroke, the inside diameter should not exceed that for a new liner by the value shown in table below (Source: Wartsila. For MAN engines the values might be slightly different)
- From about mid-stroke to the bottom of the liner, the inside diameter should not exceed that for a new liner by the value shown in the table below (Source: Wartsila. For MAN engines the values might be slightly different)
- O-ring grooves must be in good condition
Very important – Cylinder liners which exceed the above mentioned diameter limits should be scrapped!
In conclusion, in order to run the engine efficiently and to avoid engine breakdowns and damage, proper maintenance and periodical checks must be carried out as per your company and manufacturer’s maintenance plan and instructions. Moreover, every engineer, especially senior engineers and juniors who aim for promotion, must be familiar with proper monitoring, inspections and how to interpret the findings as they offer valuable information about engine running condition.
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